The Late Pleistocene–Holocene sedimentary architecture of the Southeast (SE) Vietnam Shelf was investigatedusing high-resolution seismic profiles and core samples. Three systems tracts and a prominent seismic reflectionsurface at the base of the sequence were revealed. This surface (SB1) is interpreted as a sequence boundaryformed by subaerial processes during the Late Pleistocene sea-level fall and subsequentmarine reworking duringtransgression. A surface map of the lowstand surface, compiled from seismic profiles and sediment cores, re-vealed the W–E to N–S oriented incised-valley system of the paleo-Mekong River. The incised valleys show aclear change in morphology from the north to the south in the study area. The northern incised-valley systemoff Vung Tau appears as a narrow and deep V-shape in cross-section (b5 kmwide and tens ofmeters deep) likelyas a result of the high-gradientmorphology of the paleo-shelf. In contrast, thewide and low-gradient paleo-shelf offthemodernMekongDelta and CaMau Peninsula created shallow incised-valleys (5–15 kmwide and b15 mdeep)on the exposed shelf. The lowstand systems tract (LST) consists of a prograding outer shelf delta-wedge formedduring the Last Glacial Maximum (LGM) sea-level lowstand period. The transgressive systems tract (TST) waswell preserved in the incised-valleys, where its thickness reaches 15–25 m. Sediment core analysis results and seis-mic facies reveal that the TST deposits within the incised-channels were marked by a transition from fluvial de-posits at the base to marine deposits in the upper part of the channels. On the exposed shelf and the interfluvialarea of the incised-channels, the TST is a sandy layer overlying the sequence boundary SB1. Thickness of the TSTon the shelf varies from 0 to 15 m. The highstand systems tract (HST) consists of thickmud clinoforms of themod-ernMekong subaqueous delta. TheHSTwedge prograded onto the shelf primarily after themid-Holocene sea-levelhighstandwas at approximately 6.5–5.5 kyr BP ago. The HSTwedge extends along the southwestern shore, and itsmaximum thickness (30 m) was recorded in the Cape Ca Mau area. The HST wedge pinches out at modern waterdepths of 20–30 m, resulting in a thin HST layer on the middle and outer shelf. The proposed post-Pleistocenesequence-stratigraphic model for the SE Vietnam Shelf is a variation on the theoretical model of Vail (1987). Thethick highstand wedge on the SE Vietnam Shelf is confined to the inner shelf due to the broad and low-gradientshelf morphology and the strong local hydrodynamic conditions driven by the monsoon system. Except for theone deposited within the incised-valley system, the TST deposits on the SE Vietnam Shelf tend to disperse overthe shelf instead of forming a thick backstepping unit. The accommodation space was probably created fasterthan the sediment supply during the rapid transgression.
The Southeast (SE) Vietnam Shelf is situated on the southwesterncontinental margin of the South China Sea (SCS) (Fig. 1). During theLast Glacial Maximum (LGM) and deglacial periods, the SE VietnamShelf experienced different states of exposure andflooding, correspond-ing to regressive and transgressive cycles as a result of sea-level fluctu-ations. Such variations strongly influenced the sedimentary architectureof the continental shelf, which reflects the interactions between
gy and Geophysics Vietnameseiet, Cau Giay, Hanoi, Vietnam,
g).
ghts reserved.
sediment supply, hydrodynamic conditions and sea-level change. Stud-ies of the sedimentary architecture of the shelf can thus reveal charac-teristics of the governing factors and paleo-processes and improve theunderstanding of land–ocean interactions, especially in the scope ofthe future development of Vietnam's coastal zone. Previous researchon the Late Quaternary sedimentation on the SE Vietnam Shelf mainlyfocused on the Mekong Delta plain and its Holocene evolution(Nguyen et al., 2000; Ta et al., 2001a, 2001b, 2002a, 2002b; Hori andSaito, 2007; Tamura et al., 2007, 2009). These studies concluded thatthe modern Mekong Delta was initiated approximately 8 kyr BeforePresent (BP), prograded seaward approximately 250 km fromCambodia to the SCS during the last 6 kyr and switched from a tidedominated to a mixed tide-wave dominated delta during the last3 kyr. Recent studies of the SE Vietnam Shelf have focused on theHolocene sedimentation and have suggested that the sediment
Fig. 1.Map of the Southeast Vietnam Shelf with seismic profiles. The lower smallmap shows the locations of the seismic profiles (black lines) and sediment cores (gray circles) used in thisresearch. The land elevation data is extracted from Shuttle Radar Topography Mission (SRTM) digital elevation models (http://srtm.usgs.gov).
157B.V. Dung et al. / Global and Planetary Change 110 (2013) 156–169
accumulation rate on the SE Vietnam Shelf is in the ranges of 5–10 or25–40 cm/kyr in low hydrodynamic regime areas (Schimanski andStattegger, 2005).
New results derived from hydro-acoustic surveys on the SE VietnamShelf reveal numerous asymmetric NE–SWoriented bedforms that are afew meters high and hundreds of meters long and are found in waterdepths of 20–40 m (Kubicki, 2008; Bui et al., 2009). Geomorphologyof the modern Mekong subaqueous delta was investigated and dividedinto four zones based on different characteristics of seismic progradingclinoforms and sedimentary structure (Xue et al., 2010). Modern sedi-mentation and morphology of the subaqueous Mekong Delta are alsodescribed by Unverricht et al. (2013–in this issue).
Detailed studies of the Late Quaternary sequence stratigraphy of thecontinental shelf in neighboring areas have been conducted in thecentral Sunda Shelf in the south (Hanebuth et al., 2000, 2002, 2003;Hanebuth and Stattegger, 2004). Numerous sequence stratigraphic re-sults of the SE Vietnam Shelf consider only the Cenozoic basin evolutionand the oil and gas potential. Research of the Late Quaternary sequencestratigraphy on the SE Vietnam Shelf was mainly focusing on coreanalysis results taken within the northeastern incised-valleys of the SEVietnam Shelf where transgressive deposits were well preserved(Tjallingii et al., 2010). This study revealed the sharp variation of sedi-mentary facies from fluvial to fully marine-dominated facies in theincised-valleys. A rapid flooding of the shelf between 9.5 ka and 8.5 kacorresponding to Melt Water Pulse (MWP) 1C was indicated by coreanalysis of the paleo-Mekong valley infilling and coastal depositsin the Cambodian lowlands (Tjallingii et al., 2010). However, theevolutional history of the entire shelf since the Last Glacial Maximum(LGM) remains poorly understood so far. In this study, we will present
new results from a seismic-sequence stratigraphic analysis of the SEVietnam Shelf, with an emphasis on the post-Pleistocene period. Ourresearch relies mainly on analysis of dense high resolution seismic re-flection data which more or less cover the entire SE Viet Nam Shelf(Fig. 1). The specific aims of this study are:
(1) to investigate the Late Pleistocene and Holocene seismic-sequence stratigraphic architecture on the SE Vietnam Shelf,
(2) to reconstruct and propose a sequence stratigraphic modelframework for the SE Vietnam Shelf from the LGM to the Present,and
(3) to compare theVietnamesemodel to other sequence stratigraph-ic models to infer local controlling factors.
2. Regional setting
The SE Vietnam Shelf is bordered by a 600-km long coastline to thewest, the South China Sea (SCS) to the east, the Gulf of Thailand to thesouthwest and the Sunda Shelf to the south (Fig. 1). The geological his-tory of the SE Vietnam Shelf is characterized by Cenozoic rifting phasesthat are considered to be related to the opening of the SCS, which oc-curred approximately 32 to 15.5 Ma ago and was caused by the India–Eurasia plate collision (Taylor and Hayes, 1983; Briais et al., 1993). TheSE Vietnam Shelf (between 0 and 200 m deep) is narrow in the north(~80 km wide) and widens to the south and southwest (~450 kmwide) (Fig. 1). This transition is a result of the hinterland morphology,which is dominated by high rocky mountains in the north and thelow-gradient area of the modern Mekong Delta plain in the south(Fig. 1). The land elevation data shown in Fig. 1 is extracted from Shuttle
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Radar Topography Mission (SRTM) digital elevation models (http://srtm.usgs.gov). The climate and hydrodynamic conditions of the studyarea are driven by the East Asianmonsoon,which blows from thenorth-east during winter (November to April) and from the southwest duringsummer (June to September) (Pham, 2003). The tide regime on the SEVietnam Shelf is generally considered to be non-uniform. From VungTau to the Ca Mau Peninsula, semi-diurnal tides are well developed,whereas the southwestern area off the Ca Mau Peninsula and the Gulfof Thailand are dominated by diurnal tides. The tide amplitude variesfrom 2.5 to 3.8 m in the northeastern area off Vung Tau to 0.5–1 m to-wards the Ca Mau Peninsula and the Gulf of Thailand (Wolanski et al.,1996; Nguyen et al., 2000; Pham, 2003). The Mekong River is the mostimportant sediment source for the SE Vietnam Shelf. It begins in the Ti-betan Plateau and runs through six other countries before reaching thesea in Vietnam. The river has annually suspended sediment dischargesof approximately 160 million tons/yr; the highest proportion occursfromAugust to October (rainy season) and the discharge decreases sub-stantially from February to May (dry season) (Milliman and Syvitski,1992; Nguyen et al., 2000; Pham, 2003).
3. Methods and available data
For this research, high resolution 2D seismic reflection data wereused. These data were obtained during cruises along the VietnamShelf in the framework of the Vietnam–German cooperation project:SO 140 (Wiesner et al., 1999), VG5 (2004), VG9 (2005), SO187(Wiesner et al., 2006) and Mekong (2007, 2008). Seismic data were ac-quired using a Boomer and a Parasound acoustic source. Because thefocus of this research was the shelf, the seismic profiles were locatedmostly at water depths between 0 and 200 m (Fig. 1).
Parasound is a hull-mounted system that combines a narrow-beamechosounder and a sub-bottom profiler. The system is operated with afixed primary frequency of 18 kHz and a secondary primary frequencythat varies from 20.5 to 23.5 kHz. Both primary frequencies are trans-mitted simultaneously in a narrow beam (~5°), and the constructiveinterference of these frequencies (parametric effect) is used to generatea working frequency (secondary frequency) within the beam of2.5–5.5 kHz (Grant and Schreiber, 1990). In this study, the Parasounddata were collected with a secondary primary frequency of 22 kHzand a secondary working frequency of 4 kHz. The data were digitallyrecorded and sampled at a frequency of 40 kHz. Navigation data weresupplied by the ship's global positioning system (GPS). The Boomer sys-tem (EG&G Uniboom) is a single-channel system that includes an elec-trical energy supply and an electromagnetic transducer to transform thedischarged energy to electro-dynamically generated acoustic pulses.During the surveys, the transducer of the Boomer source was used in acatamaran that was towed along with a hydrophone-streamer receiverwith 8 hydrophones astern of the vessel. The average speed of the vesselwas 4 kn. The boomer source produces a wide bandwidth working fre-quency,with amain range of 0.3 to 11 kHz. Thiswide bandwidth resultsin a typical penetration of 20 to 100 m below the seabed, depending onthe nature of the sub-seafloor strata. The Boomer source regularlyproduces 2 to 3.67 shots per second. The sound waves are reflectedwhen they reach reflection surfaces, which are regarded as acoustic-impedance contrast boundaries. The hydrophone-streamer receivesthe pressure reflection signals and converts them into voltage responsesbefore transmitting them to a computer. Seismic traces were digitallyrecorded and displayed usingNWC software. A GPSwas used to guaran-tee accurate positioning of the recorded seismic traces. During the finalcruise (Mekong 2008),we deployed a C-Boom system,which had a sim-ilar working mechanism to the EG&G Uniboom system. The dominantworking frequency of the C-Boom system is 1.76 kHz.
For data processing, a high/low pass frequency filter was applied tothe recorded data. Highpass filters are used to maintain all frequencieshigher than a selected frequency. In contrast, lowpass filters are usedto maintain all frequencies lower than a selected frequency.
Frequency band-pass filters of 2.5–6 kHz and 0.5–7 kHz were ap-plied to the Parasound and Boomer data, respectively. The key seismicreflection surfaces were then interpreted using the software KingdomSuite SMT 8.4. Average sound velocities of 1500 m/s in sea water and1550 m/s in subsurface sediments were assumed for two-way traveltime (TWT)-depth conversion.
Seismic data were interpreted on the basis of the sequence strati-graphic concept proposed by Mitchum and Vail (1977) and Vail(1987) and further refined by other authors. The seismic unitswere dis-tinguished from one another by their reflection continuity, amplitude,frequency and configuration (Fig. 2). The classification of seismic faciesand related depositional environments is adapted from Badley (1985),Vail (1987) and Veenken (2007). The interplay between base levelchanges, resulting from the combined effects of eustasy, tectonics, sed-iment compaction, and environmental energy, and the sedimentationrate controls the formation of sequence systems tracts (Fig. 3). For sim-plicity, the energy of waves and currents was neglected, and the baselevel was set to sea-level (Catuneanu, 2002). Hence, the concept ofbase level change was identical to the relative sea-level change. Accom-modation is defined as the space available for sediments to accumulate;variations in accommodation space are controlled by changes in baselevel. In this research, we applied the four-fold division of systems tractsto divide the sedimentary architecture into different stages in relation tosea-level fluctuations (Catuneanu, 2002; Catuneanu et al., 2009).
(1) The falling stage systems tract (FSST)was formed entirely duringthe stage of relative sea-level fall (forced regression), and it oc-curs independent of the ratio between the sedimentation rateand accommodation spaces.
(2) The lowstand systems tract (LST) was formed during the sea-level lowstand and slow sea-level rise when the rate of rise waslower than the sedimentation rate (normal regression).
(3) The transgressive systems tract (TST) was formed during thestage of relative sea-level rise when the rate of rise was higherthan the sedimentation rate.
(4) The highstand systems tract (HST) was formed during the latestage of relative sea-level rise when the rate of rise was lowerthan the sedimentation rate.
4. Results
4.1. Seismic units and bounding surfaces
In general, five seismic units and three major surfaces were identi-fied from the seismic profiles (Table 1). The seismic units and boundingsurfaces are named by increasing number in order of decreasing age.
Major bounding surfaces:(1) SB1 is marked by highly continuous and strong amplitude
reflections in all seismic profiles (Figs. 4, 5, 7 and 8). It canbe traced across the shelf from water depths between 20and 150 m. Farther landward (0–20 m deep), the appear-ance of shallow gas effects prevented its detection (Fig. 8).
(2) The RS1 surface mostly occurs within the incised-valley sys-tems, where it is characterized by moderate amplitudeand continuous reflections in the seismic profiles (Figs. 5, 6and 7). In the upper edge of the incised-valley systems, it isalmost merged with the lower SB1 surface (Figs. 5 and 6).The RS1 surface is recorded in the seismic profiles as theboundary between the lower strong-chaotic reflector unitand the upper planar transparent reflector unit.
(3) RS2 is the first surface that appears below the modern sea-bed (Figs. 5 and 8). In the seismic profiles of the middleand outer shelf, it is characterized by medium to low ampli-tude and highly continuous reflectors. On the inner shelf
Fig. 2. Classification of seismic facies and related depositional environments.Adapted from Badley (1985), Vail (1987) and Veenken (2007).
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around the modern Mekong subaqueous delta (0–30 mdeep), it is clearly recorded in seismic sections as a high am-plitude and continuous surface, which forms the boundarybetween the lower sheet-like transparent reflector unit andthe overlying seaward dipping reflector unit (Fig. 8).
Seismic units:(1) U1 is characterized by seaward dipping reflectors with tan-
gential oblique shapes that flatten towards the top (Fig. 4).It is recorded only on themodern outer shelf at water depthsof ~150 m. The estimated thickness of this unit in the seismicprofiles is approximately 50 m (Fig. 4).
Fig. 3. Sequence stratigraphic systems tracts, as defined by the interplay between base level chaimentation rate was assumed to be constant during base level fluctuations.
(2) U2 is recorded only within the incised-valley systems. Its de-posits prevail in the basal part of the channels. It is represent-ed by high amplitude and low continuity reflections (Figs. 5and6). The top of this unit is overlain by a smooth and contin-uous surface (RS1). In some channels, the U2 unit occupiesthe entire channel and it is overlain directly by the RS2 sur-face (Fig. 5). The maximum thickness of this unit reaches~30 m in the northern incised channel branch off Vung Tau,where the incised-valleys cut deeply into the underlying sed-iments (Figs. 5 and 6). By contrast, its thickness decreases toapproximately 10 m towards the southern area off theMekong Delta and the Ca Mau Peninsula (Fig. 7).
nges and the sedimentation rate (modified fromCatuneanu, 2002). For simplicity, the sed-
Table 1Systems tracts, seismic units and facies, bounding surfaces and reflection patterns in the SE Vietnam Shelf. Abbreviations: LST = lowstand systems tract, TST = transgressive systemstract, HST = highstand systems tract.
Systemstract
Seismicunit
HST
TST
LST
U5
U4
RS2
U3
RS1
U2
U1
SB1
Acoustic faciesand configuration
Occurence Thickness[m]
Seismic image
High to moderate amplitude,
low frequency, high continuity
oblique tangential downlap
Transparent, parallel
Low to moderate
amplitude, transparent,
parallel
Moderate to amplitude,
variable frequency, low
continuity, locally transparent,
parallel to oblique, infill
High amplitude, high
frequency, high continuity,
oblique tagential downlap,
toplap
Inner shelf
Mid-outer
shelf
Entire shelf
Incised-
channels
Outer shelf
0−25
0−1
0−15
0−30
50
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(3) U3 shows a low tomoderate amplitude reflection,with trans-parent facie developing across the shelf (Figs. 4, 5, 7 and 8).Within the incised-valleys, it occurs at the top of theincised-valleys overlying the U2 unit and has a clear bound-ary (RS1 surface). Outside the incised-valley system, thisunit is composed of a transparent layer directly overlyingthe SB1 surface (Fig. 8). This unit is bounded on the top by acontinuous and low-amplitude reflection (Fig. 5). The thick-ness of this unit is highly variable. In some incised-valleys
Fig. 4. Seismic profile (a) and sequence stratigraphic interpretation (b) of the LGM lowstand wLGM sea-level (Hanebuth et al., 2009) as a reference. Core ages are cited from Schimanski and
or on the inner shelf, its thickness reaches up to 15 m(Figs. 7 and 8), but the thickness decreases quickly to a fewmeters or is almost absent in the interfluvial area of theincised-valleys and on the middle-outer shelf (Figs. 5 and 6).
(4) U4 is the uppermost unit recorded in the seismic profiles. Itrepresents a thin layer that develops only on the middleand outer shelf. It is characterized by transparent seismic fa-cies overlying the U3 unit (Fig. 5). The presence of this unitcannot be determined where its thickness is less than the
edge deposits. The upper part of the wedge was probably reworked and eroded, using theStattegger (2005). For abbreviations, see Table 1.
Fig. 5. Seismic profile (a) and sequence stratigraphic interpretation (b) of the northern incised-valleywith different tributaries. The core results indicate a transition from fluvial (Fl) in thelower to fully marine influence (Ma) in the uppermost part (Tjallingii et al., 2010). For abbreviations, see Table 1.
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resolution of the seismic system, and it is mixed with thelower U3 unit (Figs. 6 and 7).
(5) U5 is well recorded in the inner shelf off the modernMekong subaqueous delta (0–30 m). It appears as high-amplitude, high-continuity and seaward dipping reflec-tions. It forms a tangential downlap to the lower RS2 sur-face (Fig. 8). This unit shows a seaward thinning trendwith an average thickness of 15–20 m. This unit nearlypinches out at modern water depths of 20–30 m (Figs. 8,10 and 11). Farther seaward, it is merged with the under-lying U4 unit.
Fig. 6. Seismic profile (a) and sequence stratigraphic interpretation (
4.2. Sedimentary characteristics and ages of deposits
Radiocarbon dating of sediment cores taken in the upper part of theouter shelf wedge (Fig. 4) indicated 14C ages of 24.3 kyr BP in the lowerpart of the core and 0.66 to 1.22 kyr BP in the upper part of the core(Schimanski and Stattegger, 2005). These ages correspond to the LGMsea-level lowstand and the deglacial period, according to the regionalsea-level curve (Hanebuth et al., 2000, 2009; Stattegger et al., 2013–inthis issue). Sediment cores taken from the incised-valleys at waterdepths between 29 and 60 mon the SE Vietnam Shelf show a clear tran-sition between the lower fluvial to the upper marine dominated sedi-mentation (Fig. 5; Tjallingii et al., 2010). This interpretation is based
b) of the northern incised-valley. For abbreviations, see Table 1.
Fig. 7. A. Seismic profile (a) and sequence stratigraphic interpretation (b) of the southern incised-valley off modern Mekong Delta. The top of the channel is covered by numerous activesandwaves. For abbreviations, see Table 1. B: Seismic profile (a) and sequence stratigraphic interpretation (b) of the southernmost incised-valley off CaMau Peninsula. For abbreviations,see Table 1.
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on X-ray fluorescence (XRF) core scanning results which utilize the rel-ative ratio of element Titanium and Calcium as indicator of terrigenousandmarine influences in sedimentation. The calibrated ages of deposits
of six sediment cores taken at water depths between 29 and 155 m onthe SE Vietnam Shelf range from 0.3 to 13.3 kyr BP (Tjallingii et al.,2010). The uppermost parts of eight sediment cores taken at modern
Fig. 8.A. Part of the seismic profile showinghighstandwedgedeposits (U5) of themodernMekongDelta. Highstand deposits are terminated atwater depth of 30 m. Seismic profile (a) andsequence stratigraphic interpretation (b). For abbreviations, see Table 1. B: Seismic profile showing highstand wedge deposits (U5) of the modern Mekong Delta in the area off Ca MauPeninsula. Similar to previous profile, the thick Highstand deposits are terminated at water depth of 30 m and further seaward their formation cannot be distinguished with the lowertransgressive deposits. Seismic profile (a) and sequence stratigraphic interpretation (b). For abbreviations, see Table 1.
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water depths between 50 and 150 m on the SE Vietnam Shelf show ahigh content of sand fraction, up to 97%, and carbonates, up to 19%(Schimanski and Stattegger, 2005). 14C ages of modern Mekong sub-aqueous delta deposits at water depths between 10 and 16 m rangefrom 0.05 to 3.12 kyr BP (Xue et al., 2010).
4.3. Continental shelf architecture
In general, the sedimentary architecture on the SE Vietnam Shelffrom the LGM to Present is composed of three systems tracts: thelowstand systems tract (LST), the transgressive systems tract (TST)
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and the highstand systems tract (HST). These systems tracts are bound-ed by the sequence boundary at the base and are separated by floodingsurfaces. A summary of the systems tracts and bounding surfaces islisted in Table 1.
4.3.1. Sequence boundary and flooding surfacesSB1 is a clear erosional surface forming the base of the incised-valley
system, which was created by subaerial processes or fluvial incisionduring falling of base level. It is therefore interpreted as a sequenceboundary. SB1 surface probably formed during multiple phases ofthe sea-level fall (regression stage) and sea-level lowstand and wasreworked again during transgression. Based on the geometry the sur-face and ages of the overlying deposits, the formation of the SB1 surfaceon the SE Vietnam Shelf can be correlated to the Late Pleistocene soilsurface which was dated at 25–30 kyr BP on the outer Sunda Shelf(Hanebuth and Stattegger, 2004). An interpolation map of the SB1 sur-face, derived from seismic profiles and sediment cores on land, is shownin Fig. 9. Depths below present sea-level of the Late Pleistocenelowstand surface of sediment cores on land are exacted frompublish lit-eratures (Hoang, 2002; Ta et al., 2002b). In the nearshore areas, thedepth of the lowstand surface is approximately 20 m and it is deepenedseaward. The morphology of the lowstand surface is similar to the con-figuration of the modern bathymetry. The most interesting feature ofthis map is the incised-channel system, which was probably formedduring the relative sea-level fall period. Channels can be traced by seis-mic profiles from 20 to 60 m of modern water depths and showW–E inthe north to N–S in the south orientations (Figs. 5, 6 and 7). The forma-tion of the incised-valley system seems to be controlled by the shelfmorphology, and a clear change is apparent from the northern to thesouthern part of the study area. The northern incised-valley system offVung Tau shows a narrow and deep incised V-shape (b5 km wide andtens of meters in depth), which likely resulted from the narrow andhigh-gradient morphology of the shelf (Figs. 5 and 6). In contrast, thesouthern incised-channels show an N–S orientation, with a decreasingtrend of deep incision (b15 m deep) compared to the northern ones.
Fig. 9.Morphology of the last glacial lowstand surface, with reference to the present sea-level.surface, identified from sediment cores on land.
This trend is attributed to the lower gradient of shelf morphology inthe south (Fig. 7).
The deposits of the U2 and U3 units are bounded above by the RS1surface, and the transition from the fluvial to the marine sediments isclear (Tjallingii et al., 2010). 14C dating values of U2 and U3 units at awater depth of 60 m range from 13.3 to 11.9 kyr BP (Fig. 5), corre-sponding to the transgressive period according to the regional sea-level curve (Hanebuth et al., 2000, 2009). Therefore, the RS1 surfacewas interpreted as a ravinement surface that resulted from erosion ofthe lower fluvial deposits by shallow marine hydrodynamic activityduring the transgression.
The maximum flooding surface (Mxfs) is normally defined as adownlapping surface formed at the top of transgressive deposits(Catuneanu, 2002). The RS2 surface in our research acts as a boundaryof the upper downlapping clinoforms and the lower sheet-like depositson the inner shelf (Fig. 8). It is interpreted as the Mxfs indicating theboundary between the TST and HST.
4.3.2. Lowstand systems tract (LST)The U1 unit consists of reflectors with a progradational oblique
wedge-shape, which could be attributed to the deposits of the LST.The thick tangential oblique-shape clinoforms are often associatedwith a period of sea-level stillstand or a small rise in the substantial sed-iment supply (Vail, 1987). Sediment core dating results (~24.3 kyr BP)from the upper part of the wedge (Fig. 4) suggest that it was probablyformed during the sea-level lowstand period. Therefore, the U1 unitwas interpreted as a deposition from the paleo-Mekong Delta causedby the falling stage and slow sea-level rise during the latest sea-levellowstand. As documented by the large time gap in the dating resultsalong the sediment core and the absence of topset reflectors on top ofthewedge (Fig. 4), we speculate that the lowstandwedgewas probablyreworked and eroded during the following early transgressive phase.Numerous relict sandwaves found at a water depth of 120 m on thenorthernmost part of the shelf (Fig. 12) confirm the presence of strongbottom current activities during early transgression (Bui et al., 2009).
Black numbers next to plus signs indicate depths below present sea-level of the lowstand
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The strong hydrodynamic activity during that period could have erodedthe upper part of the LSTwedge. The boundary between the LST and theTST within the lowstand wedge is not resolved under the resolution ofthe seismic data (Fig. 4). However, the age control from core SO 14093-3 suggests that the LST wedge was overlain by a veneer of TST andHST deposits.
4.3.3. Transgressive systems tract (TST)The U2 and U3 units are presentedwithin the incised-valleys and on
the exposed shelf. The deposits of the U2 andU3units showa clear tran-sition from fluvial to marine deposits, as documented by XRF core anal-ysis (Fig. 5; Tjallingii et al., 2010). From sediment cores taken atmodernwater depths between 29 and 155 m on the SE Vietnam Shelf, the agesof the U2 and U3 units were determined to be between 13.3 and 9 kyrBP (Tjallingii et al., 2010). Therefore, the U2 and U3 units were probablyformed during succeeding periods of trangression. U2 is recorded at thebase of the incised-valley with high amplitude and low continuity seis-mic reflectors, indicating possible formation in a fluvial environment.
Sediment cores can only reach the upper part of this unit, and theoldest age is approximately 13.3 kyr BP (Fig. 5), though the lower de-posits might have been formed during the lowstand period (Zaitlinet al., 1994). The U2 unit was interpreted as lowstand to transgressivefluvial deposits. The planar-transparent seismic reflectors of the U3unit suggest that it might have been formed in a low energy deposition-al environment. The uppermost parts of the sediment cores taken atmodern water depths between 50 and 150 m on the SE Vietnam Shelfshow a dominance of the sand fraction and a high content of carbonate(Schimanski and Stattegger, 2005), and correspond to the formation oftheU3 deposits in our research. Similarly, amarine carbonate sand layerwas indentified on top of all sediment cores taken from the incised-valleys on the SE Vietnam Shelf (Tjallingii et al., 2010). U3 is interpretedas transgressive estuarine to shallowmarine sand deposits. Outside theincised-valley, the U3 unit mostly appears as a parallel and transparentlayer overlying the lowstand erosional surface (Figs. 5, 6, 7 and 8).
4.3.4. Highstand systems tract (HST)The U5 unit is recorded in the seismic profiles as prograding
clinoforms that downlap onto the maximum flooding surface (MxfsRS2 surface) (Fig. 8). In the lower part, a horizontal and transparentlayer directly covers the SB1 surface. Recent studies on the delta plainnear the Cambodian lowland have indicated that the age of the Mxfsis approximately 8.0 kyr BP; thus, it would have preceded the formationof themodern Mekong Delta (Hori and Saito, 2007; Tamura et al., 2007,2009). Ages of unit U5 in our research are equivalent to the modernMekong subaqueous delta deposits which were dated between 0.05and 3.12 kyr BP (Xue et al., 2010). Similar prograding clinoformscan be found in other regions (Liu et al., 2004) suggesting that unitU5 represents the modern Mekong Delta clinoforms that developedafter the maximum mid-Holocene sea-level highstand approximately6–5.5 kyr on the SE Vietnam Shelf (Ta et al., 2002a, 2002b; Michelli,2008; Stattegger et al., 2013–in this issue). This clinoformwedge is con-fined to the inner shelf and has an offshore extension that is approxi-mately 20 km (Fig. 11).
Outside the thick highstand wedge, the HST deposit appears as athin, transparent layer (U4 unit) (Fig. 5) and it can often not bediscerned from the lower U3 unit due to the resolution of the seismicsystem (Figs. 4, 6, 7 and 8b). The U4 unit was documented in the upper-most parts of all sediment cores from the SE Vietnam Shelf as fully ma-rine deposits, with ages ranging from 0.3 to 8.0 kyr BP (Fig. 5; Tjallingiiet al., 2010). The formation of the U4 unit can be correlated to the thinmarine mud (condensed section) of the Sunda Shelf, which was datedat 11.0–4.0 kyr BP (Hanebuth and Stattegger, 2004). Therefore, the U4unit is interpreted as a condensed section that resulted from sedimentstarvation in the distal part of the HST.
4.4. Late Pleistocene–Holocene sequence stratigraphic model for the SE
4.4.1. Vietnam Shelf and controlling factorsIn general, the evolution of the SE Vietnam Shelf over the last 26 kyr
from the LGM to the Present can be divided into three periods that cor-respond to three systems tracts: LST, TST and HST.
As a result of the falling of the sea-level, which took place betweenMarine Isotope Stage 5e (~120 kyr BP) and the LGM (~20 kyr BP)(Hanebuth and Stattegger, 2004), most of the SE Vietnam Shelf was ex-posed to subaerial processes and river incision. During the LGM period,the lowstand erosional surface and incised-valleys (paleo-MekongRiver) developed on the exposed shelf as a result of base level adjust-ment. In this research, the incised-valleys could only be traced in mod-ern water depths of 20 to 60 m. In the other parts of the shelf, theincised-valleys were probably reworked and erased during the subse-quent transgression. The development of the LGM surface on the SEVietnam Shelf correlates well to the modern bathymetry of the area,with a high gradient in the north and a gentle surface towards thesouthern part (Fig. 9). The river mouths during the LGM periodwere lo-cated at depths of approximately 123 ± 2 m (Hanebuth et al., 2009)and formed a thick prograding wedge (U1 unit) on the outer shelf(Fig. 4). As discussed in the previous section, the LST wedge was partlyeroded during the onset of transgression, and this could have signifi-cantly reduced its original thickness (Fig. 4). The LST wedge thicknessof ~50 m recorded in the seismic profile evidences the stability of thesea-level for a considerable length of time and indicates a substantialsediment supply from the paleo-rivers to the outer shelf during theLGM period.
The LST deposits on the SE Vietnam Shelf consist of a progradingdelta wedge and the lowermost fluvial deposits in the incised-channels.
The termination of the LGM is complex and remains a subject of dis-cussion. In general, a slight rise of sea-level from 26 to 20 kyr BP (Peltierand Fairbanks, 2006) and the first significant sea-level rise starting from19.6 kyr BP (Fig. 13) mark the final phase of the LGM period (Hanebuthet al., 2009). The maximum flooding surface of the Mekong Delta plainwas dated around 8.0 kyr BP (Hori and Saito, 2007; Tamura et al.,2007, 2009). Hence, we tentatively infer that the TST period on the SEVietnam Shelf occurred from 19.6 to 8.0 kyr BP. Different surfaces with-in the TST, as ravinement surfaces (RS1), can be clearly observed withinthe incised-valleys, marking the transition from a fluvial to amarine en-vironment (Figs. 5 and 6). The TST on the SE Vietnam Shelf mostly ap-pears as a thin (0–5 m in thickness), sheet-like layer overlying thelowstand erosional surface (Figs. 5, 6 and 8). However, its thickness in-creases locally to approximately 15 m, with aggradational stacking pat-terns in sheltered areas with an increasing sediment supply (Fig. 8).Thick TST deposits are well recorded only within the incised-valley sys-tem at water depths between 20 and 60 m because they were well pre-served in valley depressions from the succeeding marine erosionalprocesses (Figs. 5, 6 and 10). The transgressive surface that acts as aboundary between the LST and the TST deposits in the standardincised-valleymodel (Zaitlin et al., 1994) is not visible. The preservationpotential of these facies is normally low due to the erosional processesof the transgression period (Catuneanu, 2002).
As mentioned above, the maximum flooding surface on the SEVietnam Shelf was dated at approximately 8.0 kyr BP, coincident withthe initiation of the modern Mekong Delta. After the initiation phaseof aggradation, the modern Mekong Delta prograded rapidly followingthe mid-Holocene sea-level highstand of approximately 1.5 to 2.5 mabove the modern level which reached between 6.5 and 5.5 kyr BP(Ta et al., 2002a, 2002b; Michelli, 2008; Stattegger et al., 2013–in thisissue). The highstand wedge (U5) recorded in our research is themodern Mekong subaqueous delta. This wedge mostly developedaround the modern coastline and pinches out seaward at waterdepths of 20–30 m (Figs. 8, 11 and 12).
On the middle and outer shelf, the HST appears as a thin layer over-lying the TST deposits (Figs. 4 and 5). The distribution of modern HST
Fig. 10. Thicknessmapof deglacial/Holocene sediments. The sediment depocenter is locatedmostlywithin the incised-valleys, themodernMekong subaqueousdelta and thenarrowoutershelf off Phu Qui Island. The thickness of the northern Mekong subaqueous delta was roughly calculated from sediment cores on land and bathymetry.
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deposits can be explained by two controlling factors: shelf morphologyand modern hydrodynamic conditions on the SE Vietnam Shelf. Thesouthward widening of the SE Vietnam Shelf area has increased the dis-tance (average about 200 km) between sediment sources (MekongRiver) to themiddle and outer shelf and, therefore, reduced themodernsediment supply to this area. Moreover, numerous NE–SW orientedsand waves found at modern water depths of 20–40 m suggest theactivity of strong modern bottom currents (0.6–0.7 m/s), which can
Fig. 11. Thickness of modern highstand sediments (last 8 kyr), constructed from seismic profilelocated off Cape Ca Mau, and the wedge tends to develop towards the Gulf of Thailand to the w
prevent the deposition of fine sediments (Kubicki, 2008; Bui et al.,2009). Numerous modern sand waves on the middle shelf (Figs. 7Aand 12) indicate the dominance of sand-sized fractions in the surfacesediments and a limited extension of modern fine sediments to thisarea of the shelf. The double-effect of shelf morphology and strongcurrents driven by the local monsoon system (NE–SW) might haveprevented the HST mud wedge from developing farther offshore. Inaddition, the long-shore transport of sediments from Mekong River
s. The northern part of the HST is not shown on the map. The HST sediment depocenter isest.
Fig. 12. Distribution of late Pleistocene-Holocene depositional systems in the SE Vietnam Shelf, constructed from seismic profiles. The red line shows the modern HST wedge boundary,identified from seismic profiles. The northern boundary of the HST wedge is roughly indicated by the thickness map shown in Fig. 10.
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mouths in the southwestward direction was promoted. The sedimentisopach map of the HST, constructed from seismic profiles, indicatesthat the sediment depocenter of the HST wedge is located off CapeCa Mau and it tends to develop farther westward into the Gulf ofThailand (Fig. 11).
5. Discussion
The proposed sequence-stratigraphicmodel for the SEVietnamShelfbasically follows the main features of the theoretical model of Vail(1987) (Fig. 13). Nevertheless, there are some differences betweenthese two models that reflect the local controlling factors. In general,sedimentation on the SE Vietnam Shelf reflects the interactions be-tween sea-level change, the shelf gradient, hydrodynamic conditionsand sediment supply.
On the SE Vietnam Shelf, the thick highstand wedge is confined tothe inner shelf and does not extend to the middle and outer shelf, asproposed in the Vail model. Sediment deposits on the middle andouter shelf of the SE Vietnam Shelf are typical for the modern HST star-vation shelf, which were also observed on the neighboring Sunda Shelf(Hanebuth and Stattegger, 2004) and on the shelf of the East China Sea(Dong and Soo, 2000). In general, no clear thick backstepping stackingpatterns of the TST deposits have been observed on the SE VietnamShelf except for the one deposited within the incised-valley system.We deduced that the high dispersion of the transgressive depositson the SE Vietnam Shelf was a result of a high accommodation space/sediment supply ratio, which resulted from rapid transgression overthe wide and low-gradient continental shelf. In such cases, the TST de-posits tend to be dispersed widely over the shelf instead of forming athick stack of backstepping layers (Cattaneo and Steel, 2003). Therefore,the model of thick backstepping TST deposits in the Vail model cannotbe applied to the SE Vietnam Shelf. The same is true for the sequencestratigraphic models of the Sunda Shelf (Hanebuth and Stattegger,2004), North Yellow Sea (Liu et al., 2004) and East China Sea (Dong
and Soo, 2000). The transgressive surface (TS) that separates theLST and the TST was not clearly documented on the SE VietnamShelf because it was frequently reworked and amalgamated with thelowstand sequence boundary during the transgression (Catuneanu,2002; Cattaneo and Steel, 2003). Further, the LST fluvial deposits thatare predicted to be preserved in the lowermost part of the incised-channels are neither clearly distinguished in the seismic profiles nordocumented in the sediment cores.
6. Conclusions
The Late Pleistocene–Holocene sedimentary architecture of the SEVietnam Shelf was revealed on the basis of seismic-sequence strati-graphic analysis. Five seismic units and three main bounding surfaceswere identified in the formation of the uppermost sequence of the SEVietnam Shelf.
The LSTwith unit U1 consists of a prograding outer shelf deltawedgewith a lowstand erosional basal surface. The lowstand surface is widelyrecorded over the shelf. An interesting feature of the lowstand surface isthe incised-valley system, formed by incision of the paleo-MekongRiverduring the falling sea-level. These channels are apparent in seismic pro-files in modern water depths of 20 to 60 m, and they seem to begoverned by the paleo-shelf morphology. The northern incised-valleybranch off Vung Tau has a narrow and deep V-shape in cross-section,which is likely a result of the high gradient morphology of the paleo-shelf. The infill of the northern valley with valley morphology was de-scribed by Tjallingii et al. (2010), but their formation was not linked tothe surrounding shelf deposits. In contrast, the wide and low gradientshelf off the modern Mekong Delta and the Ca Mau Peninsula createdshallow incised-valleys on the exposed shelf.
The TST was mostly preserved from marine erosional processes inthe incised-valley depressions, which are filled by unit U2. It shows anupward transition from fluvial to fully marine conditions as a result ofmarine transgression. On the exposed shelf and the in interfluvial
Fig. 13. Late Pleistocene–Holocene sequence stratigraphicmodel for the SE Vietnam Shelf (A)with the regional sea-level curve (B) (Stattegger et al., 2013–in this issue) and comparison totheoretical models of Vail (C) and Zaitlin (D).
168 B.V. Dung et al. / Global and Planetary Change 110 (2013) 156–169
zone, the TST is a thin layer thatmostly consists of sand (unit U3) direct-ly overlying the sequence boundary SB1.
The HST is primarily composed of the thick prograding mudclinoforms of the modern Mekong subaqueous delta that form unitU5. This unit is limited to modern water depths of 0–30 m. After escap-ing from the rivermouth, theHSTmud is transported along the shore tothe southwestern part and forms a sediment depocenter off Cape CaMau. On the middle and outer shelf, the HST sediments form the con-densed section,with unit U4 as a thin layer that cannot be distinguishedwith the lower transgressive sediments and has no clear boundary. Theinteractions between shelf morphology, sediment supply, local hydro-dynamic conditions and sea-level changes are important controllingfactors of the depositional systems on the SE Vietnam Shelf.
Acknowledgments
This work was completed as a result of the Vietnam–German coop-eration project funded by Deutsche Forschungsgemeinschaft (DFG) andMinistry of Science and Technology (MOST) Vietnam. Phung Van Phachthanks NAFOSTED of Vietnam for supporting this study (grant 105.01-2010.15). The authorswish to thank Dr. Gert JanWeltje (Delft Universi-ty) and Dr. RikTjallingii (NIOZ) for valuable comments on earlier ver-sions of the manuscript. Comments from Marc De Batist greatlyimproved the quality of the paper.
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